Rotary diamond dressing tools impart the required form onto a grinding wheel and must be designed and made to specifications driven by the design of the grinding wheel. These tools have narrow quality specifications with low tolerances for deviations in geometry and mechanical attributes. Although dressing tools have been constructed in a variety of ways utilizing various materials and processes, most processes known in the art are demanding and inefficient.
For example, in one commercial process, diamond grains are hand set into a pattern in the cavity of a mold with an adhesive, then a powdered metal bond material is added and pressed into place around the diamonds. The pressed materials are densified by processes such as infiltration, hot pressing, sintering, or a combination thereof, to fix the diamonds in place and form the tool. In another typical process, a diamond layer may be set onto a custom designed mold and fixed in place by reverse electroplating. See, e.g., U.S. Pat. No. 4,826,509. The sintering or plating step is followed by an extensive grinding step to remove grain high spots and to flatten the surface.
In another process described in U.S. Pat. No. 4,805,586, the diamond grains are pretreated to roughen and enlarge their surface area and to permit the grains to be arranged within the bond so that the majority of the grains are in direct contact with adjacent grains. These pretreated diamond grains are then electroplated to the surface of a base body with nickel or cobalt or alloys of nickel or cobalt.
In U.S. Pat. No. 5,505,750, the diamond grains and metal powder bond are infiltrated with a near-eutectic copper-phosphorus composition during sintering.
Many powder metal matrix abrasive components for dressing tools utilize relatively small diamond grains (e.g., less than 0.5 mm in diameter) embedded within the powder matrix and the resulting composite is ground to the required geometry. Such abrasive components are not very sharp and grinding wheel dressing with them is relatively inefficient due to rapid wear of the tool. When such a powder matrix is used with large diamond grains, the finishing process loses considerable amounts of diamond as the composite is ground to the required geometry. It is not possible to achieve a durable, fine (e.g., about 0.127 mm (0.005 inch)) dressing tip radius in tools made from diamond grains in a powder metal bond.
Polycrystalline diamond (PCD) inserts have been used to construct rotary dressing tools. PCD inserts are embedded in a powder metal matrix, sintered onto the tool, and then ground to the required geometry and surface finishing. See, e.g., U.S. Pat. No. 4,685,440. PCD inserts offer a relatively flat surface and can be easily ground to the required geometry during finishing operations, or, for some shapes, can be provided as a near net shape piece. However, PCD is not 100% diamond. PCD material initially contains significant quantities (10-12 wt %) of metal catalyst and the metal catalyst is typically leached from the PCD material, leaving voids, to yield essentially pure diamond with a density of about 90 to 95% of the theoretical density. Therefore, dressing tools made with PCD inserts lack the durability of dressing tools made with diamond abrasive grains which are fully dense, 100% diamond materials.
The rotary diamond tool for dressing abrasive wheels described in U.S. Pat. No. 5,058,562 is made by using a chemical vapor deposition (CVD) process to deposit a layer of diamond film directly onto a base plate of the tool and assembling the base plate with a pair of backup plates to provide stiffness. With this approach, there are no diamond cutting points created, merely a hard, flat diamond surface. In a dressing tool, a flat diamond surface merely acts to crush the wheel face, rather than to cut bond and spent abrasive grains from the face and, thereby, open the face of the wheel for further grinding.
The rotary diamond tool for dressing abrasive wheels described in U.S. Pat. No. 4,915,089 is made by forming a single layer of diamond grains in a plane orthogonal to the rotational axis of the tool. The layer of diamond grains is sandwiched between two layers of metal backup plates. The diamond layer is bonded to the plates by hot pressing the diamond grains and metal powder between the metal backup plates in a suitable mold to sinter the metal powder. The U.S. Pat. No. 4,915,089 mentions an alternative design wherein diamond grains are attached to one or both sides of the tool by plating or metal bonding, but teaches that the alternative design suffers the disadvantage of poor diamond retention. In the preferred design, arcurate segments of the laminated assembly of diamond grains and plates are brazed to the circumference of a disc-shaped metal wheel to form a dressing tool, optionally with a continuous abrasive rim. However, consistent with the geometry of this tool design, the patent teaches that the tool is used to dress a straight face wheel and the tool would not be useful for dressing a profile into the face of a grinding wheel.
EP-B-116668 discloses a dressing tool having a single layer of electroplated diamond grains arranged in a geometric design similar to that of the tool of U.S. Pat. No. 4,915,089. In contrast to the active braze bond used in the tools of the invention, with the electroplated bond of the EP-B-116668 tool, poorer diamond grains retention, shorter tool life and higher manufacturing costs are predicted.